Diffractive projection apparatus

ABSTRACT

A light projection apparatus is provided comprising: a source of light; a switchable grating on a first substrate; and a diffractive optical element. Light is diffracted at least once by the switchable grating and is diffracted at least once by the DOE.

REFERENCE TO EARLIER FILINGS

The present patent application is a continuation of a U.S. patentapplication having Ser. No. 13/998,799 filed on Dec. 11, 2011, which isa continuation of a U.S. patent application having Ser. No. 13/317,468filed on Oct. 19, 2011 now U.S. Pat. No. 8,639,072 which issued on Jan.28, 2014, which is the national phase application of PCT application No.PCT/GB2010/000835 filed 27 Apr. 2010, claiming priority to U.S.provisional patent application 61/202,996 filed on 27 Apr. 2010, all ofwhich are incorporated herein by reference in their entireties.

This application incorporates by reference in their entireties: PCTApplication No.: US2008/001909, with International Filing Date: 22 Jul.2008, entitled LASER ILLUMINATION DEVICE; U.S. patent application Ser.No. 10/555,661 filed 4 Nov. 2005 entitled SWITCHABLE VIEWFINDER DISPLAY;and PCT Application No. US2006/043938 with International Filing Date: 13Nov. 2006, entitled METHOD AND APPARATUS FOR PROVIDING A TRANSPARENTDISPLAY.

BACKGROUND OF THE INVENTION

This invention relates to a wearable dip lay device, and moreparticularly to a wearable display using electrically switchableholographic optical elements.

There is a requirement for a compact see through data display capable ofdisplaying image content ranging from symbols and alphanumeric arrays tohigh-resolution pixelated images. The display should be highlytransparent and the displayed image content should be clearly visiblewhen superimposed over a bright background scene. The display shouldprovide full colour with an enhanced colour gamut for optimal datavisibility and impact. A prime requirement is that the display should beas easy to wear, natural and non-distracting as possible with a formfactor similar to that of ski goggles or, more desirably, sunglasses.The eye relief and pupil should be big enough to avoid image loss duringhead movement even for demanding military and sports activities. Theimage generator should be compact, solid state and have low powerconsumption.

The above goals are not achieved by current technology. Current wearabledisplays only manage to deliver see through, adequate pupils, eye reliefand field of view and high brightness simultaneously at the expense ofcumbersome form factors. In many cases weight is distributed in theworst possible place for a wearable display, in front of the eye. Themost common approach to providing see through relies on reflective ordiffractive visors illuminated off axis. Microdisplays, which providehigh-resolution image generators in tiny flat panels, do not necessarilyhelp with miniaturizing wearable displays because the requirement forvery high magnifications inevitably results in large diameter optics.Several ultra low form factor designs offering spectacle-like formfactors are currently available but usually require aggressivetrade-offs against field of view, eye relief and exit pupil.

The optical design benefits of DOEs are well known including unique andefficient form factors and the ability to encode complex opticalfunctions such as optical power and diffusion into thin layers. Bragggratings (also commonly termed volume phase grating or holograms), whichoffer the highest diffraction efficiencies, have been widely used indevices such as Head Up Displays.

An important class of diffractive optical element known as anelectrically Switchable Bragg Gratings (SBG) is based on recording Bragggratings into a polymer dispersed liquid crystal (PDLC) mixture.Typically, SBG devices are fabricated by first placing a thin film of amixture of photopolymerizable monomers and liquid crystal materialbetween parallel glass plates. One or both glass plates supportelectrodes, typically transparent indium tin oxide films, for applyingan electric field across the PDLC layer. A Bragg grating is thenrecorded by illuminating the liquid material with two mutually coherentlaser beams, which interfere to form the desired grating structure.During the recording process, the monomers polymerize and the PDLCmixture undergoes a phase separation, creating regions densely populatedby liquid crystal micro-droplets, interspersed with regions of clearpolymer. The alternating liquid crystal-rich and liquid crystal-depletedregions form the fringe planes of the grating. The resulting Bragggrating can exhibit very high diffraction efficiency, which may becontrolled by the magnitude of the electric field applied across thePDLC layer. In the absence of an applied electric field the SBG remainsin its diffracting state. When an electric field is applied to thehologram via the electrodes, the natural orientation of the LC dropletsis changed thus reducing the refractive index modulation of the fringesand causing the hologram diffraction efficiency to drop to very lowlevels. The diffraction efficiency of the device can be adjusted, bymeans of the applied voltage, over a continuous range from essentiallyzero to near 100%. U.S. Pat. No. 5,942,157 by Sutherland et al. and U.S.Pat. No. 5,751,452 by Tanaka et al. describe monomer and liquid crystalmaterial combinations suitable for fabricating SBG devices.

There is a requirement for a compact, lightweight wearable displayproviding a high brightness, high contrast information display with ahigh degree of transparency to external light

SUMMARY OF THE INVENTION

The objects of the invention are achieved in one embodiment in whichthere is provided a wearable display comprising a light-guide formingone transparent substrate of an HPDLC cell and a Diffractive OpticalElement (DOE) forming the second transparent substrate. The twosubstrates together function as a light guide. The inside surfaces ofeach substrate are patterned with ITO to provide a set of SBGs. Each SBGdevice contains information encoded in a multiplicity of separatelyswitchable grating regions. Said regions may be information symbols.Alternatively, the SBGs may be configured to provide two dimensionalpixelated arrays. In each case the SBGs are confined to symbol or pixelregions, the display being perfectly transparent elsewhere. Guided lighthitting a particular SBG region is diffracted towards the viewer andoverlaid on the background scene while light missing the symbolundergoes TIR. Applying an electric field across a given symbol erasesit from view. Said SBG and said DOE together form a magnified image ofthe symbols or pixels.

In a one embodiment of the invention the DOE is a transmission element.

In a one embodiment of the invention the DOE is a reflection element.

In one embodiment of the invention there is provided a wearable displaycomprising a light-guide forming one transparent substrate of an HPDLCcell and a DOE forming the second substrate and further comprising alaser illuminator.

In one embodiment the laser illuminator comprises red, green and bluelaser sources, a beam combiner and expander, a means for minimizinglaser speckle and a means for coupling illumination to the curved lightguide.

In a one embodiment of the invention said second substrate is a curvedtransparent element with no optical power.

In one embodiment of the invention, the wearable display is configuredto provide symbols of different colors by arranging for differentsymbols to contain SBGs optimized for the required wavelengths and LEDsof appropriate spectral output.

In one embodiment of the invention several SBG panels could be stackedsuch that by selectively switching different layers it is possible topresent a range of different symbols at any specified point in the fieldof view.

In one embodiment of the invention several SBG panels each design tooperate a specific wavelength could be stacked such that by selectivelyswitching different layers it is possible to present different coloursat any specified point in the field of view.

In one particular embodiment of the invention there is provided awearable display comprising first and second substrates sandwiching aHPDLC region. A diffractive lens is applied to a first region of theouter surface of the first substrate. A diffractive mirror is applied toa second region of the outer surface of the first substrate. The twosubstrates together function as a light guide. The inside surfaces ofeach substrate are patterned with ITO to provide a set of SBGs. Theouter surfaces of said first substrate faces the eye of the viewer ofthe display. Each SBG device contains information encoded in amultiplicity of separately switchable grating regions. Said regions maybe information symbols. Alternatively, the SBGs may be configured toprovide two dimensional pixelated arrays. In each case the SBGs areconfined to symbol or pixel regions, the display being perfectlytransparent elsewhere. Guided light hitting a particular SBG region isdiffracted towards the viewer and overlaid on the background scene whilelight missing the symbol undergoes TIR. Applying an electric fieldacross a given symbol erases it from view. The SBG and DOE together forma magnified image of the symbols or pixels.

In one embodiment of the invention there is provided a wearable displaycomprising first and second substrates sandwiching a HPDLC region. Thetwo substrates together function as a light guide. A first holographicmirror is applied to the outer surface of the first substrate. A quarterwave plate is disposed adjacent to the outer surface of the secondsubstrate. A second holographic mirror is disposed adjacent to thequarter wave plate. The inside surfaces of each substrate are patternedwith ITO to provide a set of selectively switchable SBG regions. EachSBG device contains information encoded in a multiplicity of separatelyswitchable grating regions. Said regions may be information symbols.Alternatively, the SBGs may be configured to provide two dimensionalpixelated arrays. In each case the SBGs are confined to symbol or pixelregions, the display being perfectly transparent elsewhere. Guided lighthitting a particular SBG region is diffracted towards the viewer andoverlaid on the background scene while light missing the symbolundergoes TIR. Applying an electric field across a given symbol erasesit from view. Said SBG and said DOE together form a magnified image ofthe symbols or pixels.

In any of the above embodiments the substrates sandwiching the HPDLClayer may be planar, curved or formed from a mosaic of planar or curvedfacets.

In one embodiment of the invention there is provided a pixelated edgelit wearable display in which the SBG regions combine the functions ofcoupling light from the TIR path and imaging said light onto the retina.The eyeglass display comprises a two-dimensional array of independentlyaddressable SBG regions where each SBG region has a unique opticalprescription designed such that input collimated light incident in afirst direction is deflected into output collimated light propagating ina second direction towards the eye. The SBG layer is sandwiched betweentransparent substrates. The substrates and SBG array together form alight guide. ITO layers are applied to the opposing surfaces of thesubstrates with at least one ITO layer being patterned such that SBGregions may be switched selectively. Input light is scanned andmodulated by a laser scanning system and injected into the eyepiecewhere it performs TIR until diffracted out of the eyepiece towards theeye by one or more active SBG regions. Portions of the field of view aresequentially imaged onto the retina by switching groups of SBG regionsin sequence and scanning rays with a predetermined range of incidenceangles onto the SBG group while the SBG regions comprising the group arein their active state The region of active SBG regions may comprise arectangular area.

In one embodiment of the invention said group of SBG regions is providedby a rectangular sub array of SBG regions.

In one embodiment of the invention said group of SBG regions is providedby a sequence of SBG regions disposed along a row or column of SBGregions, said row or column being activated in a scrolling fashion.

A more complete understanding of the invention can be obtained byconsidering the following detailed description in conjunction with theaccompanying drawings, wherein like index numerals indicate like parts.For purposes of clarity, details relating to technical material that isknown in the technical fields related to the invention have not beendescribed in detail.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side elevation view of a portion of a wearabledisplay in one embodiment of the invention.

FIG. 2 is a schematic side elevation view of a portion of a wearabledisplay in one embodiment of the invention.

FIG. 3 is a schematic side elevation view of a portion of a wearabledisplay in one embodiment of the invention.

FIG. 4 is a schematic side elevation view of a portion of a wearabledisplay in one embodiment of the invention.

FIG. 5 is a schematic side elevation view of a portion of a wearabledisplay in one embodiment of the invention.

FIG. 6 is a chart illustrating the diffraction efficiency versusincident angle of an SBG in the state in which no electric field isapplied to the SBG.

FIG. 7 is a schematic side view of the exposure system used to createthe SBG.

FIG. 8A is a side elevation view of a stage in the manufacture of acurved display element.

FIG. 8B is a side elevation view of a stage in the manufacture of acurved display element.

FIG. 8C is a side elevation view of a stage in the manufacture of acurved display element.

FIG. 8D is a side elevation view of a stage in the manufacture of acurved display element.

FIG. 8E is a side elevation view of a stage in the manufacture of acurved display element.

FIG. 8F is a side elevation view of a stage in the manufacture of acurved display element.

FIG. 9 is a schematic plan view of laser illuminated wearable displayprovided by another embodiment of the invention.

FIG. 10 is a schematic side elevation view of a portion of a wearabledisplay in a further embodiment of the invention.

FIG. 11 is a schematic side elevation view of a portion of a wearabledisplay in a further embodiment of the invention.

FIG. 12 is a schematic three-dimensional view of a particular embodimentof the invention.

FIG. 13 is a schematic side elevation view of a particular embodiment ofthe invention.

FIG. 14 is a schematic plan view of a particular embodiment of theinvention.

FIG. 15 is a schematic three-dimensional view of a detail of aparticular embodiment of the invention.

FIG. 16 is a schematic side elevation view of another embodiment of theinvention.

FIG. 17 is a three dimensional schematic view of one embodiment of theinvention using an SBG array.

FIG. 18 is a schematic illustration showing the embodiment of FIG. 17 inmore detail

FIG. 19 is a front elevation view of one embodiment of the inventionusing an SBG array.

FIG. 20 is a front elevation view of a first operational state of theembodiment of FIG. 19.

FIG. 21 is a front elevation view of a second operational state of theembodiment of FIG. 19.

FIG. 22 is a front elevation view of a third operational state of theembodiment of FIG. 19.

FIG. 23 is a three dimensional schematic view of one embodiment of theinvention using an SBG array.

FIG. 24 a schematic illustration showing the embodiment of FIG. 23 inmore detail.

FIG. 25A is a side elevation view of a first operational state of theembodiment of FIG. 23.

FIG. 25B is a side elevation view of a second operational state of theembodiment of FIG. 23.

FIG. 25C is a side elevation view of a third operational state of theembodiment of FIG. 23.

FIG. 26A is a schematic plan view of an optical system for use in theembodiment of the invention.

FIG. 26B is a schematic side elevation view of an optical system for usein the embodiment of the invention.

FIG. 27 is a schematic illustration showing parameters of the human usedin the description of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be further described by way of example only withreference to the accompanying drawings.

FIG. 1 shows a schematic side elevation view of a portion of oneeyepiece of a wearable display in one embodiment of the invention.Although a planar element is shown the complete eyepiece may have acurved or facetted surface. The portion of the display shown in FIG. 1comprises a DOE 10 an HPDLC layer comprising flood cured regionindicated by 20 surrounding at least one independently switchable SBGregion indicated by 30 and a transparent substrate layer 40. The HPDLClayer is sandwiched between the substrate and the DOE. Said SBG regionsmay be information symbols. Alternatively, the SBG regions may beconfigured to provide two dimensional pixelated arrays. In each case theSBGs are confined to the symbol or pixel regions the display beingperfectly transparent elsewhere. The SBG and DOE together encode thecharacteristics of a lens whose function will be explained below.

A set of transparent electrodes, which are not shown, is applied to bothof the inner surfaces of the substrates. The electrodes are configuredsuch that the applied electric field will be perpendicular to thesubstrates. Typically, the planar electrode configuration requires lowvoltages, in the range of 2 to 4 volts per μm. The electrodes wouldtypically be fabricated from Indium Tin Oxide (ITO). The light guidelayer and DOE 10 and 40 together form a light guide. The grating region30 of the SBG contains slanted fringes resulting from alternating liquidcrystal rich regions and polymer rich (ie liquid crystal depleted)regions. In the OFF state with no electric field applied, theextraordinary axis of the liquid crystals generally aligns normal to thefringes. The grating thus exhibits high refractive index modulation andhigh diffraction efficiency for P-polarized light. When an electricfield is applied to the SBG, the grating switches to the ON statewherein the extraordinary axes of the liquid crystal molecules alignparallel to the applied field and hence perpendicular to the substrate.Note that the electric field due to the planar electrodes isperpendicular to the substrate. Hence in the ON state the gratingexhibits lower refractive index modulation and lower diffractionefficiency for both S- and P-polarized light. Thus the grating region 12no longer diffracts light towards the eye and hence no symbol isdisplayed. Each symbol is selectively controlled by an independent pairof planar electrodes. Typically, the electrode on one substrate surfaceis uniform and continuous, while electrodes on the opposing substratesurface are patterned to match the shapes of the said SBG regions.Desirably, the planar electrodes should be exactly aligned with the SBGregions for optimal switching of the SBG regions and the elimination ofany image artifacts that may result from unswitched SBG regions.

Turning now to FIG. 2 we consider the operation of the light guide inmore detail. In FIG. 2 the display is again illustrated in a schematicside view. It will be seen that the display further comprises, an inputlight guide 70, and beam stop 80. The SBG region sandwiched between theDOE and the second substrate comprises at least one grating region 30and flood cured regions 20 a, 20 b on either side of the SBG gratingregion. The grating region has a first surface facing the viewer and asecond face. The input lightguide 70 is optically coupled to thesubstrates 10 and 40 such the light from the LED undergoes totalinternal reflection inside the lightguide formed by 10 and 40. Lightfrom the external scene, generally indicated as 500 propagates throughthe display towards the viewer. The propagation of light from the sourcethrough the display may be understood by considering the state when theSBG is diffracting, that is with no electric field applied. The rays 101and 102 emanating from the light source 60 are guided initially by theinput lightguide 70. The ray 102, which impinges on the second face ofthe grating region 30, is diffracted out of the display in the direction201 towards the viewer. A virtual viewable image of the dataholographically encoded in the SBG region is formed by the combinedaction of the SBG and the DOE. On the other hand, the rays 101 which donot impinge on the grating region 30 will hit the substrate-airinterface at the critical angle and are totally internally reflected inthe direction 103 and eventually collected at the beam stop 80 and outof the path of the incoming light 500.

Referring now to FIG. 3 we consider the formation of the viewable imagein one embodiment of the invention. The rays 102 which impinge on theSBG region 30 are diffracted towards the viewer. The SBG regioncorresponds to an off axi holographic lens which when illuminated by theoff axis input rays 102 forms a virtual image 400 behind the display, ieon the opposite side of the display from the viewer. The virtual raysfrom the virtual image 400 are generally indicated by 200. The combinedaction of the SBG and the DOE forms an exit pupil indicated by EP at adistance from the display indicated by ER with the limiting rays beinggenerally indicated by 300. It should be noted that the final image neednot be at infinity. In certain cases, a comfortably viewable image maybe provided at a closer distance. Either the DOE or the SBG may havediffusing properties in addition to the basic lens characteristicsrecorded therein.

The DOE is designed to perform two functions. Firstly, the DOE forms avirtual image at infinity in conjunction with the SBG. Secondly the DOEcompensates for aberrations and distortions created by the SBG. The SBGand DOE together encode the optical prescription of a diverging asphericoff-axis lens. It should be noted that the DOE is designed to haveminimal diffraction efficiency for ambient light transmitted through thedisplay. A DOE can be designed and fabricated for high diffractionefficiency for a single wavelength use using a classical Fresnel lensapproach. A DOE may also be designed for operation with a discretenumber of wavelengths, in which case the DOE is a multi-order orharmonic DOE. The master DOE element may be fabricated usingconventional multilevel lithography to achieve optimum diffractionefficiency and replicated by plastic injection molding for massproduction injection molding.

FIG. 4 shows a schematic side elevation view of another embodiment ofthe invention. Again a portion of one eyepiece of a wearable display isillustrated. The display comprises a DOE mirror 11 an HPDLC layer 20containing the SBG 30 and a substrate layer 41. The SBG encodes thecharacteristics of a lens. The DOE encodes the characteristics of amirror. The HPDLC layer 20 is sandwiched between the substrate and theDOE. The substrate and DOE provide transparent substrates for the HPDLClayer. A set of transparent electrodes, which are not shown, is appliedto both of the inner surfaces of the substrates. FIG. 5 is a schematicside elevation view of a portion of the display illustrating theformation of an image. The SBG deflects the incoming guided rays 110 toform a virtual image indicated by 410 located in front of the display.The reflective DOE then magnifies the virtual image 410 giving a finalimage at “infinity” indicated by the rays 510. The combined action ofthe SBG and the DOE forms an exit pupil indicated by EP at a distancefrom the display indicated by ER. The virtual ray paths from the virtualimage 410 are generally indicated by 210, while the rays defining thepupil are generally indicated by 310. The final image need not be atinfinity. In certain cases a comfortably viewable image may be providedat a closer distance. Either the DOE or the SBG may have diffusingproperties in addition to the basic lens characteristics recordedtherein. The advantage of the using a reflection DOE is that it causesless disturbance of ambient light due to the inherently narrow bandwidthof reflection gratings. Another advantage the embodiment of FIGS. 4-5 isthat the reflection DOE may have substantial optical power. In opticaldesign terms reflective DOEs provide more degrees of freedom foroptimizing diffraction efficiency with respect to illumination lightwhile minimizing the diffraction of external light. Advantageously,reflection DOEs can be optimized to provide high diffraction efficiencyat low incidence angles.

FIG. 6 is a chart illustrating the diffraction efficiency versus angleof an SBG grating in the OFF state. This particular grating has beenoptimized to diffract red light incident at around 72 degrees (the Braggangle) with respect to the normal of the substrate. The Bragg angle is afunction of the slant of the grating fringes and is chosen such that thediffracted light exits close to normal (0 degrees) to the substrate. Tomaximize the light throughput, the light source and input lightguideshould be configured such that light is launched into the lightguide atthe Bragg angle. This can be accomplished by various means well known tothose skilled in the art, including the use of lenses, gratings orprisms. Light launched into the lightguide must be at an angle greaterthan the angle for Total Internal Reflection (TIR) in order to be guidedby the lightguide. Hence, the Bragg angle must be chosen to be largerthan the angle for TIR. The invention is not restricted to anyparticular method of introducing light into the lightguide.

In order to ensure high transparency to external light, high contrast ofdisplayed information (ie high diffraction efficiency) and very low hazedue to scatter the following material characteristics are desirable. Alow index-modulation residual grating, with a modulation not greaterthan 0.007, is desirable. This will require a good match between therefractive index of the polymer region and the ordinary index of theliquid crystal. The material should have a high index modulationcapability with a refractive index modulation not less than 0.06. Thematerial should exhibit very low haze for HPDLC cell thicknesses in therange 2-6 micron. The HPDLC should have a good index match (to within+0.015) for glass or plastic at 630 nm. One option is 1.515 (forexample, 1737F or BK7 glasses). An alternative option would be 1.472(for example Borofloat or 7740 Pyrex glasses).

FIG. 7 is a schematic side elevation view of a laser exposure systemused to record the SBG grating. The exposure system comprises a prism 90mount on top of and in optical contact with the substrate 40, a mask fordefining the shapes of the symbols or pixels to be projected containingopaque regions such as 91 a and 91 b, and two mutually coherentintersecting laser beams generally indicated by 401 and 402. The prismhas a top surface substantially parallel to the substrate and angle sidefaces. The beam 401 is introduced via the top surface of the prism. Thebeam 402 is introduced via a side face of the prism. The mask defines anaperture through which portions of the beams can impinge on the mixtureof photopolymerizable monomers and liquid crystal material confinedbetween the parallel substrates 40 and 10. The interference of the beamwithin the region defined by the aperture creates a grating region 30comprising alternating liquid crystal rich and polymer rich regions. Theshape of the aperture defines the shape of the symbol or pixel array. Itwill be clear from consideration of FIG. 7 that a plurality of symbolsmay be created in this way. Referring again to FIG. 7 we see that theflood-cured regions 20 a, 20 b are created by the beam 402. Since thereis no intensity variation in this region, no phase separation occurs andthe region is homogeneous, haze-free and generally does not respond toapplied electric fields. Advantageously the beam inside the light guidewould have an incidence angle of 72 degrees corresponding to the Braggangle of the SBG grating.

Desirably the light sources are solid-state lasers. An exemplary laseris the NECSEL developed by Novalux Inc. (CA). The NECSEL has severaladvantages including: better directionality than laser diodes; verynarrow bandwidths and availability of red, green and blue devices. Thelow etendue of lasers results in considerable simplification of theoptics. LEDs may also be used with the invention. However, LEDs sufferfrom large etendue, inefficient light collection and complex illuminatorand projection optics. A further disadvantage with regard to SBGs isthat LEDs are fundamentally unpolarized.

The laser power requirement will depend on the required symbol tobackground contrast. A typical requirement is around 50:1 contrast. In atypical practical monochromatic display embodiment, we may assume:ambient illumination in bright daylight of 10⁴ lux; a display area of 5cm²; optical losses of 45%; and a luminous efficacy for green laserlight of 680 lumens/W. Such a display would require approximately 850 mWof green laser power.

FIGS. 8A-8F illustrate the steps in manufacturing a curved eyepiecedisplay of the type discussed above. Side elevation views of theeyepiece are shown in each case.

At step 1 a planar transparent substrate 10 is provided (FIG. 8A).At step 2 a second planar transparent substrate 40 comprising a DOE isprovided (FIG. 8B).At step 3 said first and second substrates are combined in a cell 45with spacers 46 (FIG. 8C).At step 4 the cell is mechanically deformed into a curved form 47 (FIG.8D).At step 5 the cell 47 is filled with a HPDLC mixture 48 (FIG. 8E).At step 6 an SBG 49 (FIG. 8F) is recorded into the HPDLC mixture usingtwo crossed mutually coherent laser beams.

FIG. 9 shows a schematic side elevation view of a wearable displayincorporating the elements illustrated in FIG. 2 with the source 60 ofFIG. 2 replaced by a laser illumination module. The illumination modulecomprises red green and blue lasers 60 a,60 b,60 c a beam combiner andbeam expander module 61, a despeckling device 62 and an optical means 63for coupling the laser light into the lightguide. The lasers emit redgreen and blue beams 200 a,200 b,200 c respectively. The beam combinerand expander combines beams 200 a,200 b,200 c into to a single expandedbeam 210 Speckle is a well-known problem in laser displays. Speckle canbe reduced by applying decorrelation procedures based on combiningmultiple sets of speckle patterns or cells from a givenspeckle-generating surface during the spatio-temporal resolution of thehuman eye. Desirably the despeckler 62 is an SBG device configured togenerate set of unique speckle phase cells by operating on the angularor polarization characteristic of rays propagating through the SBGdevice. The SBG despeckler device may comprise more than one SBG layer.Furthermore, the SBG despeckler device may be configured in severaldifferent ways to operate on one of more of the phase, and ray angularcharacteristics of incoming light. In one implementation of theinvention the SBG despeckler device may be configured as a diffuser. Inanother implementation of the invention the SBG despeckler device may beconfigured as a phase retarder based on a sub wavelength gratingexhibiting form birefringence. Alternatively, the SBG despeckler devicemay be configured as a lens of the type known as an axicon. Varying theelectric field applied across the SBG despeckler device varies theoptical effect of the SBG despeckler device by changing the refractiveindex modulation of the grating. Said optical effect could be a changein phase or a change in beam intensity or a combination of both. Theoptical effect of the SBG despeckler device is varied from zero tomaximum value at a high frequency by applying an electric field thatvaries in a corresponding varying fashion. Said variation may followsinusoidal, triangular, rectangular or other types of regular waveforms.Alternatively, the waveform may have random characteristics. The SBGdespeckler device may comprise similarly characterised first and secondgratings disposed in series. Each incremental change in the appliedvoltage results in a unique speckle phase cell. A human eye observingthe display integrates speckle patterns to provide a substantiallyde-speckled final image. The beam combiner 61 may comprise separate redgreen and blue SBG layers operated to diffract light from the red greenand blue lasers sequentially into a common direction towards thedespeckler. The invention does not rely on any particular despecklertechnology. Any method for generating and averaging speckle cells may beused with the invention. However, solid-state methods using SBGs orother electro-optical devices offer more scope for miniaturization ofthe illuminator module.

The optical design of a wearable display according to the principles ofthe invention will be dictated by basic geometrical considerations wellknown to those skilled in the art of optical design. The goal is tomaximize eye relief, exit pupil and field of view. Since theseparameters will impact on geometrical aberrations, dispersion and otherfactors affecting image quality some performance versus form factortrade-offs are inevitable. The preferred light source is a laser. Ifbroadband sources such as LEDs are used the design will require carefulattention to the correction of chromatic dispersion and monochromaticgeometrical aberrations. Dispersion is a problem for any DOE illuminatedby a broadband source. The degree of defocus or image blur due todispersion depends on the source spectral bandwidth and the distancefrom the DOE to the virtual image plane. Typically, the angular blur fora given wavelength and a source spectral bandwidth will be of the orderof the bandwidth divided by the wavelength. The effect of monochromaticgeometrical aberrations will depend on the field of view and pupil size.

In preferred practical embodiments of the invention the display isconfigured as a layer that may be attached to a standard pair of glassesor goggles. In such embodiments the display is essentially a long clearstrip appliqué running from left to right with a small illuminationmodule containing laser die, light guides and display drive chip tuckedinto the sidewall of the goggle. Only a standard index matched glue isneeded to fix the display to the surface of the goggles.

In a further embodiment of the invention illustrated in FIGS. 10-11 theDOE element is replaced by a transparent substrate without opticalpower. In FIGS. 10-11 the elements indicated by 12 and 42 aretransparent substrates without optical power. In this embodiment thevirtual viewable image is formed by the action of the SBG only. In allother respects the propagation of light through the display is the sameas for the embodiment illustrated in FIGS. 1-3.

FIG. 12 shows a schematic side elevation view of a portion of oneeyepiece of a wearable display in one embodiment of then invention.Although a planar element is shown the complete eyepiece may have acurved or facetted surface. The portion of the display shown in FIG. 12comprises a first transparent parallel face substrate 11 an HPDLC layercomprising flood cured region indicated by 21 surrounding at least oneindependently switchable SBG region indicated by 31 and a secondtransparent parallel face substrate layer 41. The HPDLC layer issandwiched between the two substrates. A diffractive mirror 12 isapplied to a first region of the outer surface of the first substrate. Adiffractive lens 13 is applied to a second region of the outer surfaceof the first substrate. For the purposes of explaining the inventionsaid upper and lower regions may be assumed to correspond to the upperand lower portions of the inner surface of the eyepiece as viewed by awearer of the display. Said SBG regions may be information symbols.Alternatively, the SBG regions may be configured to provide twodimensional pixelated arrays. In each case the SBGs are confined to thesymbol or pixel regions the display being perfectly transparentelsewhere. The diffractive mirror and diffractive lens together encodethe characteristics of a lens whose function will be explained below. Aset of transparent electrodes, which are not shown, is applied to bothof the inner surfaces of the substrates. The electrodes are configuredsuch that the applied electric field will be perpendicular to thesubstrates. Typically, the planar electrode configuration requires lowvoltages, in the range of 2 to 4 volts per μm. The electrodes wouldtypically be fabricated from Indium Tin Oxide (ITO). The substratestogether form a light guide. The grating region 31 of the SBG containsslanted fringes resulting from alternating liquid crystal rich regionsand polymer rich (ie liquid crystal depleted) regions. In the OFF statewith no electric field applied, the extraordinary axis of the liquidcrystals generally aligns normal to the fringes. The grating thusexhibits high refractive index modulation and high diffractionefficiency for P-polarized light. When an electric field is applied tothe SBG, the grating switches to the ON state wherein the extraordinaryaxes of the liquid crystal molecules align parallel to the applied fieldand hence perpendicular to the substrate. Note that the electric fielddue to the planar electrodes is perpendicular to the substrate. Hence inthe ON state the grating exhibits lower refractive index modulation andlower diffraction efficiency for both S- and P-polarized light. Thus thegrating region 31 no longer diffracts light towards the eye and hence nosymbol is displayed. Each symbol is selectively controlled by anindependent pair of planar electrodes. Typically, the electrode on onesubstrate surface is uniform and continuous, while electrodes on theopposing substrate surface are patterned to match the shapes of the saidSBG regions. Desirably, the planar electrodes should be exactly alignedwith the SBG regions for optimal switching of the SBG regions and theelimination of any image artifacts that may result from unswitched SBGregions. In the embodiment of FIG. 12 the SBG comprises an array ofsymbols. One such symbol is indicated by 32.

The formation of an image by the eyepiece may be understood by againreferring to FIG. 12. The display is provided with input collimatedlight generally indicated by 110. The input light is admitted by lightcoupling optics similar to that illustrated in FIG. 2. The inventiondoes not rely on any particular method of coupling input light into theeyepiece. For example, coupling optics based on components such asgratings, holograms, prisms, lens and others may be used. The inputlight propagates along a first TIR path within the light guide formed bythe substrates as indicated by the rays 111,112. It should be noted theTIR ray directions are parallel to plane normal to the substratesurfaces. The SBG symbols are configured to diffract light into thedirections generally indicated by 113 when in their diffracting state.In certain embodiments of the invention only one symbol will be view atany particular time. In embodiments of the invention where more than onesymbol is to be presented to the viewer the SBG symbols would beactivated sequentially such that while only one symbol is active at anyinstant each symbol to be presented is active for a portion of the eyeintegration time. The light from the symbol 32 strikes the diffractivemirror 12 and is deflected into a second TIR path indicated by the rays114,115. It should be noted that the first and second TIR paths arecharacterised by ray paths in orthogonal planes as indicated by FIG. 12.The rays in the second TIR path strike the diffractive lens and aredeflected towards the viewer forming an exit pupil indicated by 51.

Optical power may be encoded into one or both of the diffractive lensand diffractive mirror. Each SBG symbol is designed to diffuse incidentlight into a cone around the diffracted ray direction. For example, theSBG symbol 32 diffracts light into a cone around the diffracted raydirection 113. In certain embodiments of the invention it may beadvantageous to provide said diffusion by first fabricating a computergenerated hologram (CGH) having the required diffusion characteristicsand then recording a hologram of said CCG into the SBG symbol. The CGHwould typically be a surface relief diffractive optical element.

The diffractive mirror may be a Bragg grating, a switchable Bragggrating, a surface relief diffractive optical element, a computergenerated hologram or a mirror formed using any other type ofdiffracting structure.

The diffractive lens may be a Bragg grating, a switchable Bragg grating,a surface relief diffractive optical element, a computer generatedhologram or a lens formed using any other type of diffracting structure.

FIG. 13 is a schematic side elevation view of the embodiment illustratedin FIG. 12 showing the second TIR path in more detail. FIG. 14 is aschematic plan view of the embodiment illustrated in FIG. 12 showing thefirst TIR path in more detail.

In the embodiment of FIG. 12 the SBG symbol diffracts light into anaverage direction substantially normal to the surface of the symboltowards the diffractive mirror. The diffractive mirror is disposedopposite the SBG symbols. Light in the first TIR path is not reflectedin to the second TIR path by the diffractive mirror because the rayangles of the first TIR path are designed to fall outside the angularbandwidth within which the diffractive mirror provides efficientreflection. The rays of the first TIR path propagate through thediffractive mirror without deviation and undergo total internalreflection. On the other hand, the rays from the first TIR path that arediffracted by SBG symbols fall within the angular bandwidth within whichthe diffractive mirror provides efficient reflection and are reflectedinto the second TIR path.

In an alternative embodiment of the invention illustrated in theschematic three dimensional view of FIG. 15 the problem of interactionsbetween the first TIR path and the diffractive mirror is avoided bydisposing the mirror at a lower level than the SBG symbol arraydesigning the SBG symbols to diffract light into a downward direction.The substrates are not illustrated in FIG. 15. Light in the first TIRpath is characterised by the rays 111,112 propagating parallel to theplane 130 orthogonal to the plane of the SBG symbol array 31. The SBGsymbols diffract light downwards in the direction 117 towards thediffractive mirror 12. The diffractive mirror reflects the light 117into a second TIR path characterised by the rays 118,119. As in theembodiment of FIG. 12 the first and second TIR paths are orthogonal.

FIG. 16 shows a schematic side elevation view of a portion of oneeyepiece of a wearable display according to the principles of theinvention. Although a planar element is shown the complete eyepiece mayhave a curved or facetted surface. The portion of the display shown inFIG. 16 comprises a first transparent parallel face substrate 11 anHPDLC layer comprising flood cured region indicated by 21 surrounding atleast one independently switchable SBG region indicated by 31 and asecond transparent parallel face substrate layer 41. The HPDLC layer issandwiched between the two substrates. The substrates 11 and 41 form thewalls of an SBG cell with the inside surfaces of each substrate beingpatterned with ITO electrodes to provide a set of SBG symbols.

A first holographic mirror 14 is applied to the outer surface of thefirst substrate. A quarter wave plate 42 is disposed in contact with theouter surface of the second substrate. A second holographic mirror 43 isdisposed in contact with the quarter plate. At least one of theholographic mirrors has optical power such that a virtual image isformed behind the display ie on the opposite side to the eye.

Referring again to FIG. 16 we now consider the propagation of lightthrough the display. Incident P polarized light 140 from one or morelaser sources propagates within the light guide formed by thesubstrates. The exact configuration of the laser sources and the meansfor injecting laser light into the display does not form part of thepresent invention. The light 140 is diffracted and diffused by the SBGlayer, which is sensitive to P-polarized light, forming a divergent beamgenerally indicated by 141. The SBG directs the light 141 towards thefirst holographic mirror. The first holographic mirror is designed toreflect P light in to the beam path generally indicated by 142. Thefirst holographic mirror transmits the 1 S-polarized component ofincident light. The reflected light 142 propagates back through the SBG.It should be noted that the reflected P-polarised light 142 avoids beingdiffracted by the SBG due to its incidence angle at the SBG fallingoutside the diffraction efficiency angular bandwidth of the SBG.According to the basic optics of gratings the angular bandwidth of theSBG is considerably smaller for rays that are incident substantiallynormal to the SBG, as will be the case for light reflected from thefirst holographic mirror. After passing through the SBG for a secondtime the light 142 is converted without substantially deviation into thelight generally indicated by 143. The light 143 passes through thequarter wave plate retarder whereupon it is converted to circularlypolarized light of a first sense 144 and then undergoes reflection atthe second holographic mirror to provide the reflected light 145 whichis circularly polarised in an opposing sense to the light 144. Theholographic mirror shown in FIG. 16 has optical power resulting incollimation of the light 144. After passing through the quarter waveretarder for a second time the circularly polarised light 145 isconverted into S polarised light generally indicated by 146. TheS-polarised light 146 passes through the SBG without deviation as thelight 147 since the SBG only diffracts P-polarized light. TheS-polarised light is transmitted towards the viewer as the collimatedlight 148 by the first holographic mirror. Since the light is collimatedthe viewer is presented with a virtual image of the SBG symbols.Although the image is nominally at infinity the focal length of thesecond holographic mirror would be chosen to provide an image at somecomfortable viewing distance of around 2-3 meters. In alternativeembodiments of the invention the collimation may be provided by thefirst holographic mirror or by the first and second holographic mirrorsin combination.

The embodiment of FIG. 16 relies on optimal polarization separation bythe first holographic mirror to ensure that the P-polarised image lightfrom the SBG symbols does not get through. An additional P-blockingpolarizer may be disposed adjacent to the first holographic mirror toeliminate stray P-polarised light. It is known that holographic mirrorscan be configured to be polarization sensitive. This property has beendemonstrated in holographic mirrors recorded in dichromated gelatin(DCG) where Kogelnik theory predicts S/P separation is possible from theBrewster angle right up to around 85 degrees.

In the above-described embodiments the SBG symbol is based on overlayingan ITO pad shaped in the form of a symbol over a correspondingly shapedSBG region into which Bragg grating with diffusing properties isrecorded. Other methods of providing an SBG symbol may be used with theinvention. For example, the SBG may be of a more complex form comprisinga grating formed by a wavefront encoding the characteristics of asymbol. An SBG symbol formed in this way may allow greater control overthe characteristics of the viewable symbol. For example, the diffusioncharacteristics may be controlled. In addition, the SBG encoded opticalcharacteristics that allow the image location, image magnification andimage aberrations to be optimized. The SBG may be produced by firstdesigning mid fabricating a CGH with the required optical properties andthen recording said CGH into the SBG.

In any of the above-described embodiments the SBG could be pixilated inthe form of a two dimensional array. Such an SBG configuration would beappropriate for high information content displays.

In the embodiments to be described in the following paragraphs there isprovided a pixelated edge lit eyeglass display in which the SBG pixelsor regions combine the functions of coupling light from the TIR path andimaging said light onto the retina. The eyeglass display comprises atwo-dimensional array of independently addressable SBG regions whereeach SBG region has a unique optical prescription designed such thatinput collimated light incident in a first direction is deflected intooutput collimated light propagating in a second direction towards theeye eliminating the need for a projection lens. The SBG layer issandwiched between transparent substrates. The substrates and SBG arraytogether form a light guide. ITO layers are applied to the opposingsurfaces of the substrates with at least one ITO layer being patternedsuch that SBG elements may be switched selectively. Input light isscanned and modulated by a laser scanning system and injected into theeyepiece where it performs TIR until diffracted out of the eyepiecetowards the eye by a group of active SBG regions. Portions of the fieldof view are sequentially imaged onto the retina by switching groups ofSBGs in sequence and scanning rays with a predetermined range ofincidence angles onto the SBG group while the SBG regions comprising thegroup are in their active state The active SBG regions may cover arectangular area.

In the embodiment of the invention illustrated in FIG. 17 an eyepieceaccording to the principles of the invention comprises a two-dimensionalarray of independently addressable SBG pixels. The device comprises anarray of SBG elements 30 where each element encodes a predeterminedoptical function. The grating function may be defined in many differentbut, to a first order, may be characterised by a grating spacing and agrating vector. Typically, each SBG region has a unique opticalprescription designed such that input collimated light incident in afirst direction is deflected into output collimated light propagating ina second direction towards the eye. The SBG layer is sandwiched betweentransparent substrates 10,40. ITO layers are applied to the opposingsurfaces of the substrates with at least one ITO layer being patternedsuch that SBG elements may be switched selectively.

FIG. 18 provides a schematic illustration of the laser scanning system 1in relation to the eyepiece of FIG. 17 which is illustrated in sideelevation view. Light 300 from the laser module 2 is deflected in the Xand Y directions by the deflector 3 controlled by the XY scan drivermodule 4. The light beams are modulated by an electro optical modulationdevice 5 to provide modulated scanned light beam 310 which is injectedinto the eyepiece by means of an optical coupling element 72.

The invention does not assume any particular type of laser. Desirablythe laser comprises red green and blue emitters integrated in a compactmodule. The scanner is typically a miniature piezoelectric device.However, any other type of compact scanning device may be used with theinvention. The invention does assume any particular of modulator.Electronic circuitry for switching the SBG elements and supply power tothem is also applied to the substrates. The invention does not rely onany particular method for implementing the electronics circuitry. Theinvention does not assume any particular method for coupling the scannedlaser beam into the eyeglass. The substrates and the SBG layer togetherprovide a light guide. Illumination light from external laser RGB sourceis coupled into the eyepiece and propagates under TIR in the Y directionas in indicated in the illustration. The input laser light is scannedand amplitude modulated to provide a range of ray angles such as301,302,303 around a mean launch angle into the guide. It should benoted that the invention does not assume any particular scan pattern.

Turning again to FIG. 17 we see that at any instant groups of SBGregions such as the one indicated by 35 are activated, ie in theirdiffracting states, with all other regions generally indicated by 36being inactive ie in their non-diffractive state. All SBG regions in theactive group have substantially the same refractive index modulation. Itwill be clear from consideration of FIG. 17 that input light undergoesTIR until it impinges on an active SBG region group. During the time thegroup is active the group is illuminated by collimated incident TIRlight having a unique incidence angle. scan SBG regions in the activegroup diffract incident rays into a unique field direction as collimatedlight. For example, rays incident at the SBG group in the directions301,302,303 are diffracted into the directions 311,312,313. The eyefocuses the collimated light onto the retina to form a discrete imageregion. For example, beams in the direction 311,312,313 are focused intobeams 321,322,323 forming image points 331,332,333 on the retina.

The SBG array architecture is illustrated in more detail in FIGS. 19-22.FIG. 17 illustrates one embodiment in which the SBG array 30 used withtwo scanning modules 1A,1B each similar in principle to module 1 in FIG.18. Each scanning modules provides XY scanning according to theprinciples of FIG. 17 for TIR paths in the Y and X directionsrespectively indicated generally by 310A,310B. FIGS. 20-21 illustrateddifferent SBG configurations that may be used to provide colour imaging

In one embodiment of the invention illustrated in FIG. 20 the base setof SBG regions comprises the RGB diffracting SBG regions 97R,97G,97Bwhich are illuminated by TIR light in the direction 310A and the red,green, blue (RGB) diffracting SBG regions 97 a,97 b,97 c which areilluminated by TIR light in the direction 310B.

In one embodiment of the invention illustrated in FIG. 21 the base setof SBG regions 98 comprises the RGB diffracting SBG region 98A which isilluminated by TIR light in the direction 310A and the RGB diffractingSBG region 98B which is illuminated by TIR light in the direction 310B.In such an embodiment the RGB incidence angles must be selected suchthat in each case the peak wavelength and the incidence angle at the SBGsatisfy the Bragg condition for the required output beam angle.

In one embodiment of the invention the base set may comprise a singleSBG region 99A designed to diffract RGB light and diffract light fromlaser scanning modules 1A and 1B.

In one embodiment of the invention only one RGB laser scanner module isprovided.

In one embodiment of the invention only one RGB laser scanner module isprovided with light being piped from scanning module 1A to an opticalport located at some other edge of the eyepiece.

In one embodiment of the invention separate up/down TIR paths may be togenerate upper/lower image fields). The above light paths may beprovided by separate external light pipes from the scanner/modulator.Alternatively, the upper and right edges of the eyepiece may incorporatereflectors.

It should be clear that other methods of combining SBG switching andlaser scanning based on the principles described above may be used withthe present invention.

It will be clear from first order optical consideration that a largenumber of SBG regions must be active at any instant in order that theexit pupil is filled. To a rough approximation the size of the exitpupil should be of the order of the area of the active SBG region group.Typically, as much as 25% of the total available SBG region populationmay need to be active at any time to ensure that the exit pupil isfilled. At any instant in time all SBGs in a group have identical indexmodulation. Desirably the exit pupil is of the order of 8-10 mm indiameter. It will be clear that the number of groups, group geometry andthe number of groups that can be activated during an image frame dependson the SBG switching time, the beam scanning pattern, TIR pathlimitations imposed by the range of incidence angles and the number ofelements needed to fill the exit pupil. Typical SBG relaxation times arein the region of 500 microseconds. In one embodiment of the inventionthe array is divided into four quadrants which are switched sequentiallyduring the field time.

It will be clear form consideration of basic diffraction theory thatscanning the input light as described above allows greater imageresolution than would be possible by simply illuminating an SBG arraywith a stationary light beam. The diffraction limited angular resolutionregion size δθ of an SBG element is given by δθ−λ/d where is thewavelength and d is the aperture of a SBG region. The display field ofview FOV is given by: FOV=2*a tan (N*d/2*ER) where ER is the eye-reliefand N is the number of SBG elements. Hence the number of resolvablepixels n is given by n=FOV/δθ. For the number of resolvable pixels tomatch the number of SBG regions the value of d should be approximately√(λ*ER). If we assume WVGA resolution (480×800) and substitute thevalues ER=20 mm; λ=0.5 microns; and d=100 microns into the aboveequation the SBG array is: 48 mm×80 mm. This is too large for mostpractical eyeglass applications.

Another consequence of using a static illumination beam is that the beamcross section diffracted from an SBG region would be far too small tofill the eye pupil. The present invention overcomes this problem byusing simultaneously active groups of SBG regions to fill the pupil. Inthis sense the present invention provide what may be described as apupil expander.

A second important benefit of combining the SBG array and a scanner isthat SBG regions can be made big enough to overcome the above describeddiffraction limitations while keeping the overall array dimensionswithin acceptable form factor limits.

A third important benefit which results from being able to use largerSBG regions is that the diffraction efficiency of the region increaseswith the size of the region due to the larger number of Bragg surfacesthat interact with the incident light

The SBG regions may have more sophisticated prescriptions than the basicbeam-steering functions described above. For example, SBGs may alsoencode aspect ratio shaping, focus control and other functions.

In one embodiment of the invention the SBG array could be replaced by anarray of switchable thin gratings operating in the Raman Nath regime.

Advantageously, the SBG array fabricated using a diffractive opticalmask formed on a transparent sapphire wafer. The SBG region opticalprescriptions are defined on a region to region basis. The process offabricating the SBG array may start with the creation of a multiphasecomputer generated hologram encoding the desired optical functions whichis then holographically recorded into the SBG.

In further embodiments of the invention each SBG region may encode basicbeam steering functions required to implement the above describedembodiments together with additional optical functions includingmagnification, trapezoidal correction (that is, keystone correction) andbeam shaping. In one embodiment of the invention the SBG array regionsencode Fourier type SBG diffusers and beam shapers. In one embodiment ofthe invention the SBG array regions encode refractive microlenses. Inone embodiment of the invention the SBG array regions encode diffractiveFresnel lenses. In one embodiment of the invention the SBG array regionsencode orthogonal cylindrical diffractive lenses.

Although image modulation is provided by the laser scanner in certainembodiment of the invention the SBG be used to modulate light inassociation with the laser scanner.

Any display device using lasers will tend to suffer from speckle. Thepresent invention may incorporate any type of despeckler.Advantageously, the despeckler would be based on electro-opticalprinciples. The present invention may incorporate a despeckler based onthe principles disclosed in the PCT Application US2008/001909, withInternational Filing Date: 22 Jul. 2008, entitled LASER ILLUMINATIONDEVICE, which is incorporated herein in its entirety. The need for adespeckler may be eliminated by using a miniature, broadband (4 nm) RGBlasers of the type supplied by Epicrystal Inc.

Another embodiment of the invention directed at providing an eyeglasscombining an SBG array with a laser optical scanner within a thin edgeilluminated eye-piece will now be described.

In an embodiment of the invention illustrated in FIG. 23 an eyepieceaccording to the principles of the invention comprises a two-dimensionalarray of independently addressable SBG pixels. The device comprises anarray of SBG elements each having a predetermined grating function. TheSBG layer is sandwiched between transparent substrates 10,40. ITO layersare applied to the opposing surfaces of the substrates with at leas oneITO layer being patterned such that SBG elements may be switchedselectively. FIG. 24 provides a schematic illustration of the scanningsystem using in the embodiment of FIG. 23. The scanning system issimilar the one of FIG. 18 except that in FIG. 22 the scanning is in theX-direction only, that is, in the XZ plane.

The substrates and the SBG layer together provide a light guide.Illumination light from external laser RGB source is coupled into theeyepiece and propagates in the Y direction illustrated in the figure.The input laser light is scanned and amplitude modulated to provide arange of ray angles such as 401,402,403 around a mean launch angle intothe guide. Rows of SBGs are switched sequentially in the Y direction asindicated by 36A,36B,36C.

The principles of the embodiment of FIG. 23 may be understood byconsidering the SBG row 36A. The group of SBG regions indicated by 37are activated, ie in their diffracting states, with all other regionsbeing inactive ie in their non-diffractive state. Each region in theactive group has the same modulation. It will be clear fromconsideration of FIG. 23 that input light undergoes TIR until itimpinges on an active SBG. During the time the group is active the groupis illuminated by collimated incident TIR light having a uniqueincidence angle. SBG regions in the active group diffract incident raysinto a unique field direction as collimated light. For example, raysincident at the SBG group in the directions 401,402,403 are diffractedinto the directions 411,412,413. The eye focuses the collimated lightonto the retina to form a discrete image region. For example, beams inthe direction 411,412,413 are focused into beams 421,422,433 formingimage points 431,432,433 on the retina.

FIGS. 25A-25C provide a plan schematic view of three states of theeyepiece. Again considering the SBG row 36A, and considering the raypaths shown in FIGS. 25A-25C collectively, we see that the input TIRbeams 401,402,403 are diffracted by active SBG groups indicated by37A,37B,37C to provide diffracted beams 411,412,413 which are focused onto the retina to provide focal spots 431,432,433. The above process isrepeated for each SBG row. The width of the SBG group 37 may containaround 25% of the regions in the active row. At any instant in time allSBGs in a group have identical index modulation. The total length of thegroup of active SBG regions roughly matches the diameter of the displayexit pupil. As in the case of the embodiment of FIG. 17 the embodimentof FIG. 23 provides an expanded exit pupil but relying on scrollingrather that simultaneous activation of two dimensional SBG sub-arrays.It will be clear from consideration of FIG. 23 that the scrolling schemeillustrated therein may operate in either the Y or X directions.

A schematic unfolded plan and side elevation views of the scan opticsare provided in FIG. 26A and FIG. 26B. The apparatus comprises a lasersource 14, a scanner 15 the scanner angle magnifying lens systemcomprising the lenses 16 and 17. The front view (FIG. 26A) of theeyepiece and the side view (FIG. 26B) are generally indicated by 18. Thelaser provides a collimated output beam 500 which is scanned in the XYplane by the scanner 15. Three typical ray paths from the scanner501A,501B,501C are illustrated which after injection into the eyeglass18 follow a TIR path 502 and provide the beams indicated by502A,502B,502C. Typically, the beam cross sections are matched to thedimension of the SBG regions. The intersection of the scanned beams withSBG region columns indicated by 39A,39B. The SBG region prescriptionscontain a compensation factor to allow for the variation of theincidence angle between the first and final SBG column.

The first-order optical design parameters that apply to the embodimentsof FIGS. 17 and 23 are shown in FIG. 27. The parameter comprise: the eyeglass dimension, D; the exit pupil dimension, d, which to first order isthe same as the dimension of the active SBG region group; the eye reliefER; and the field of view half-angle U. Said parameters are related byequation: D=2*ER*tan(U)+d. The invention does not rely on any particularmethod for introducing light into the eyepiece. The coupling means couldcomprise a stack of gratings used to transmit red, green and blue lightinto the display. The gratings may be non switchable gratings. Thegratings may be holographic optical elements. The gratings may beswitchable gratings. Alternatively, prismatic elements may be used. Theeyepiece may further comprise a layer abutting the substrate 40 forproviding optical correction. Said layer may be a diffractive opticalelement or a hologram. Said layer may be used for the purposes ofmatching the eyeglasses user's spectacle prescription. The eyepiece mayfurther comprise a light control film applied to either substrate toblock stray light that would otherwise reduce contrast and degrade colorgamut. Specifically, the light control film eliminates zero order (ornon-diffracted light) and spurious diffracted light arising from thediffraction of more than one wavelength by the SBG regions. Further, thediffraction efficiency versus incidence angle characteristic oftransmission gratings will exhibit secondary diffraction maximum to bothsides of the primary diffraction peak. While the peak diffractionefficiency of these secondary peaks will be small, the amount ofundesired light extracted form the light guide may be sufficient toreduce the color purity of the display. By careful design of theincidence angles at which light is launched and the incidence angles atthe display device it is possible to ensure that light from secondarydiffraction maxima is absorbed at the light control film. One knownmeans for providing a light control film comprises an array ofmicro-sphere lenses embedded in a light-absorbing layer. Each lensprovides a small effective aperture such that incident rayssubstantially normal to the screen, are transmitted with low loss as adivergent beam while incident rays incident at an off axis angle, areabsorbed. Light control films of this type are manufactured by 3M Inc.(Minnesota). Other methods of providing a light control film, such aslouver screens may be used as an alternative to the light control filmdescribed above.

With regard to the scrolling scheme illustrated in FIG. 23, it should beunderstood that other scanning sequences, including scanning multiplecolor bands, are possible within the scope of the invention. In allcases, the position of the color bands are moved in sequential steps bymeans of selecting the voltages applied to the SB G array. It shouldalso be understood that the switching of the illumination bands must bedone in synchronism with the image modulation of the scanned laserbeams. It also must be understood that the entire sequence must berepeated at a sufficient rate that the viewer's eye merges thesequential single-colored images into a composite full-color image.

The embodiments of the invention have been described in relation totransmission SBGs. One of the known attributes of transmission SBGs isthat the LC molecules tend to align normal to the grating fringe planes.The effect of the LC molecule alignment is that transmission SBGsefficiently diffract P polarized light (ie light with the polarizationvector in the plane of incidence) but have nearly zero diffractionefficiency for S polarized light (ie light with the polarization vectornormal to the plane of incidence. Transmission SBGs may not be used atnear-grazing incidence as the diffraction efficiency of any grating forP polarization falls to zero when the included angle between theincident and reflected light is small. A glass light guide in air willpropagate light by total internal reflection if the internal incidenceangle is greater than about 42 degrees. Thus the invention may beimplemented using transmission SBGs if the internal incidence angles arein the range of 42 to about 70 degrees, in which case the lightextracted from the light guide by the gratings will be predominantlyP-polarized.

In an alternative embodiment of the invention the display device can beimplemented using reflection SBGs. Reflection gratings can be configuredto have narrow, sharply defined wavelength bandwidth, and are relativelyinsensitive to variations in angle of the light incident on the grating.The disadvantage of reflection SBG is high operating voltage. Whilereflection gratings diffract both polarization states when the includedangle between the incident and reflected light is small, the diffractionefficiency of any grating for P polarization falls to zero when theincluded angle between the incident and diffracted beams is 90 degrees.The light diffracted by a reflection grating will be predominantlyS-polarized if the angle between the incident and diffracted beams isgreater than 70 degrees. Techniques for recording reflection hologramsfor use with illumination at near-grazing incidence are known in the artand are described in U.S. Pat. No. 6,151,142. In particular, great caremust be taken during the hologram recording process to avoid reflectionsfrom the ITO electrodes and other internal surfaces within the ESBGdevices. Such undesired reflections change the fringe visibility duringthe hologram recording and may result in objectionable anduncontrollable variations of the grating diffraction efficiency. Inaddition, the refractive index of the HPLDC material during the hologramrecording process must be essentially equal to that of the glass cell.

It should be noted that the ray paths shown in FIGS. 1-25 are meant tobe schematic only. The number of total internal reflections will dependon the scrolling scheme used and the overall geometry of the light guideformed by the display layers. With regard to the embodiments describedabove, it should be noted that in order to ensure efficient use of theavailable light and a wide color gamut for the display, the SBG devicesshould be substantially transparent when a voltage is applied, andpreferably should diffract only the intended color without an appliedvoltage. It should be emphasized that the Figures are exemplary and thatthe dimensions have been exaggerated. For example, thicknesses of theSBG layers have been greatly exaggerated. In any of the embodiments ofthe invention the light sources may be lasers. Exemplary lasers includeextended cavity surface emitting laser devices such as thosemanufactured by Novalux Inc. (CA). In any of the embodiments of theinvention the light sources may be Light Emitting Diodes. Exemplary LEDsinclude devices based on photonic lattice technology such as thosemanufactured by Luminus Inc. (CA). It should be emphasized that theillumination directing device used in the above embodiments offers thebenefits of uniform illumination and reduction of the overall thicknessof the display. A key feature of all of the embodiments described aboveis that they provide the benefit of see-through. The latter is of greatimportance in Head Up Displays for automobile, aviation and othertransport applications; private see-through displays such for securitysensitive applications; architectural interior signage and many otherapplications. With the addition of a holographic brightness enhancingfilm, or other narrow band reflector affixed to one side of the display,the purpose of which is to reflect the display illumination wavelengthlight only, the see-through display can be made invisible (and hencesecure) in the opposite direction of view. Here the reflected displayillumination is effectively mirrored and therefore blocked in onedirection, making it ideal for transparent desktop display applicationsin customer or personal interview settings common in bank or financialservices settings.

Although the present application addresses wearable displays it will beclear that in any of the above embodiments the eye lens and retina maybe replaced by any type of imaging lens and a screen. Any of the abovedescribed embodiments of the invention may be used in either directlyviewed or virtual image displays. Possible applications range fromminiature displays such as those used in viewfinders to large areapublic information displays. The above described embodiments may be usedin applications where a transparent display is required. For example,the invention may be used in applications where the displayed imagery issuperimposed on a background scene such as heads up displays andteleprompters. The invention may be used to provide a display devicethat is located at or near to an internal image plane of an opticalsystem. For example, any of the above described embodiments may be usedto provide a symbolic data display for a camera viewfinder in whichsymbol data is projected at an intermediate image plane and thenmagnified by a viewfinder eyepiece. It will be clear the invention maybe applied in biocular or monocular displays. The invention may also beused in a stereoscopic wearable display. Any of the above describedembodiments of the invention may be used in a rear projectiontelevision. The invention may be applied in avionic, industrial andmedical displays. There are applications in entertainment, simulation,virtual reality, training systems and sport. Any of the above-describedembodiments using laser illumination may incorporate a despeckler devicefor eliminating laser speckle disposed at any point in the illuminationpath from the laser path to the eyeglass. Advantageously, the despeckleris an electro-optic device. Desirable the despeckler is based on a HPDLCdevice. In any of the above embodiments the substrates sandwiching theHPDLC layer may be planar, curved or formed from a mosaic of planar orcurved facets.

A wearable display based on any of the above-described embodiments maybe implemented using plastic substrates. Using sufficiently thinsubstrates such embodiments could be implemented as a long clear stripappliqué running from the nasal to ear ends of each eyeglass with asmall illumination module continuing laser dies, light guides anddisplay drive chip tucked into the sidewall of the eyeglass. A standardindex matched glue would be used to fix the display to the surfaces ofthe eyeglasses. In applications such as DSLR viewfinders the SBG symbolarray would typically be recorded using masked exposure processes.However, masked exposure may not be necessary in all applications. Anadvantage of avoiding masking processes is that the erasure of the SBGwhen it is not in its active state could be more complete. The inventorshave found that the improved erasure results from the SBG being formedover a larger area with a lower degree of modulation of the grating. Themethod of fabricating the SBG pixel elements and the ITO electrodes usedin any of the above-described embodiments of the invention may be basedon the process disclosed in the PCT Application No. US2006/043938 withInternational Filing Date: 13 Nov. 2006, entitled METHOD AND APPARATUSFOR PROVIDING A TRANSPARENT DISPLAY, which is incorporated herein in itsentirety. The transparent edge lit displays disclosed in the presentapplication may employ features disclosed in U.S. patent applicationSer. No. 10/555,661 filed 4 Nov. 2005, entitled SWITCHABLE VIEWFINDERDISPLAY which is incorporated herein in its entirety. In any of theabove embodiment of the invention, the SBG regions could be configuredto provide symbols of different colors by arranging for differentsymbols or pixels to contain SBGs optimized for the required wavelengthsand sources of appropriate spectral output. In any of the aboveembodiment of the invention, of the basic invention several SBG layerscould be stacked such that by selectively switching different layers itis possible to present different colours at any specified point in thefield of view. In any of the above embodiment of the invention, of thebasic invention several SBG layers could be stacked such that byselectively switching different layers it is possible to present a rangeof different symbols or other types of image information at anyspecified point in the field of view.

It should be understood by those skilled in the art that while thepresent invention has been described with reference to exemplaryembodiments, it is to be understood that the invention is not limited tothe disclosed exemplary embodiments. Various modifications,combinations, sub-combinations and alterations may occur depending ondesign requirements and other factors insofar as they are within thescope of the appended claims or the equivalents thereof.

What is claimed is:
 1. A light projection apparatus comprising: a sourceof light having a first wavelength; a switchable grating on a firstsubstrate; and a DOE, wherein said light is diffracted at least once bysaid switchable grating and is diffracted at least once by said DOE, 2.The apparatus of claim 1 wherein at least one of said switchable gratingor said DOE modulate said light with image information.
 3. The apparatusof claim 1, wherein said first substrate and said DOE sandwich saidswitchable grating, wherein electrodes are applied to opposing faces ofsaid first substrate and said DOE.
 4. The apparatus of claim 1, whereinsaid DOE has one of a Fresnel lens form or a multi order DOE form. 5.The apparatus of claim 1, wherein said switchable grating is a gratingrecorded in a polymer and liquid crystal material system and comprisesalternate liquid crystal rich and polymer rich regions, wherein therefractive index modulation of said grating is electricallycontrollable.
 6. The apparatus of claim 1, wherein said switchablegrating is one of a Bragg grating, a Raman-Nath grating or a subwavelength grating recorded in a holographic polymer dispersed liquidcrystal material.
 7. The apparatus of claim 1, wherein said DOE and saidswitchable grating together provide modulated light for viewing by oneof an eye or an imaging sensor.
 8. The apparatus of claim 1, whereinsaid switchable grating comprises a multiplicity of separatelyswitchable grating elements.
 9. The apparatus of claim 1, furthercomprising a substrate for modifying the polarization of said light. 10.The apparatus of claim 1, wherein said switchable grating performs atleast one of the functions of applying spatial modulation to said light,applying temporal modulation to said light, modifying the wavefrontshape of said light or modifying the spatial amplitude of said light.11. The apparatus of claim 1, further comprising a source emitting lightat a second wavelength, and further comprising a DOE and a switchablegrating designed for operation at said second wavelength.
 12. Theapparatus of claim 1, wherein said DOE or said switchable grating hasdiffusing properties.
 13. The apparatus of claim 1, wherein said DOE andsaid first substrate provide a light guide.
 14. The apparatus of claim1, wherein said DOE and said first substrate provide a light guide andlight from said source is coupled into said light guide by means of agrating or a prism.
 15. The apparatus of claim 1, wherein said DOE andsaid first substrate provide a light guide, wherein said light undergoesa reflection at a substrate to air interface.
 16. The apparatus of claim1, wherein said DOE and said first substrate provide a light guide,wherein said light traverses said switchable grating without diffractionat least once.
 17. The apparatus of claim 1, wherein said substrate andsaid DOE are curved.
 18. The apparatus of claim 1, wherein each of saidswitchable grating or said DOE is one of a transmission or reflectiongrating.
 19. The apparatus of claim 1, further comprising at least oneof a beam deflector for scanning said light in at least one of twoorthogonal directions or a modulator for amplitude modulating saidlight.
 20. The apparatus of claim 1, wherein said apparatus provides oneof a wearable display, a head up display, a viewfinder, or a projectiondisplay.